1. Field of the Invention
The present invention relates to acousto-optic devices for use with optical fibers, and particularly to an acousto-optic structure which is detachable from an optical fiber system, and which can be configured to define an acousto-optic modulator, an optical tap or a frequency shifter/mode coupler for two-mode optical fiber.
2. The Prior Art
Use of optical fiber technology in communications and remote sensing systems continues to increase, as more applications are developed. As applications in these technologies grow, so does the demand for simple, reliable components, including light modulators. For applications such as frequency shifting in fiber gyroscopes, mode-locking of fiber lasers, or switchable tapping of short-hall networks, in situ modulation of the guided light is desired as it avoids such detrimental effects as coupling losses and etalon effects.
For example, one prior art acoustic-transducer for use with optical fibers involved the transmission of an acoustic signal through a quartz block to an optical fiber embraced by the quartz block. The gap between the quartz block and the optical fiber was filled with a liquid to obtain acoustic impedance matching. This system avoided much physical damage to the fiber, but the use of the block and acoustic impedance matching fluid produced attenuation and reflections of the acoustic signal, limiting the acoustic signal frequency which could be used. Further, since the acoustic signal needed to intersect the small fiber, proper alignment of the acoustic transducer and the optical fiber through the block was critical. However, since the transducer was fixed onto the block, and since contact with the fiber was achieved by pressing a planar face of the block against the fiber, alignment of the device was very difficult during assembly. Since the block was fixed relative to the fiber after assembly, changing the alignment after assembly was virtually impossible. In addition, the use of liquid to interface the block with the fiber further complicated the system, as well as its assembly. As a result of difficulties such as these, different types of acoustic devices which eliminate the long path through the liquid are needed. At the same time other applications of this basic principle, such as modulator, have become of interest.
Demonstration of the growth of well-oriented films of zinc oxide has spawned a generation of all-fiber modulators, including phase modulators, modal couplers, and optical taps. While this technology is easily applied to the simple rectangular electrode requirements of phase modulators, more exotic devices such as mode couplers, frequency shifters and optical taps, which require transducer arrays for phase matching, are extremely difficult to fabricate. For example, these all-fiber devices require a fiber suitable for high vacuum conditions; i.e., one with its acrylate jacket completely removed or with a special jacket of metal or possibly teflon. The process of preparing the fibers in this way and of fabricating the transducer on these tiny fibers is very difficult and involves a high likelihood of fiber breakage before fabrication is complete. All-fiber devices have the additional disadvantage of interlocking of waveguide and modulator. In other words, these devices must be made on a relatively short segment of the optical fiber, and must be spliced into existing systems. This requires interruption of system operation and the splicing may result in degradation of system operation. Furthermore, device replacement requires interruption of light transmission in the system.
Unsuccessful efforts to overcome the difficulties described above have been directed to techniques such as inserting the optical fiber into an intact capillary, and using a quartz stacking wax as a coupling medium. One attempt to produce such a phase modulator is described in F. S. Hickernell, et al., Thin-Film Zinc Oxide Bulk Wave Transducer Structures for Acousto-Optic Phase Modulation of Single-Mode Fibers, Proc. IEEE Ultrasonics Symposium, 1986, pp. 715-718. In that case, no phase modulation was detected due to poor coupling.
Based on the above, it would be an important improvement in the technology to provide an acousto-optic modulator which would avoid the detrimental effects of coupling losses and etalon effects, would be relatively simple to fabricate, and which would operate at efficiencies comparable to all-fiber modulators wherein zinc oxide is deposited directly onto the optical fiber. It would be a further improvement in the technology to provide such a modulator which is a separable component from the waveguide, so that the modulator may be applied to and removed from existing fiber systems in a manner which readily permits accurate alignment of the transducer with the fiber, and which avoids interruption of light transmission and provides greater flexibility in choice of fiber. It would be still a further improvement in the technology to provide such a modulator which uses a planar transducer geometry, so as to simplify the lithography needed for more intricate electrode patterns such as those required for mode couplers, frequency shifters and optical taps.